For many years people have been dissatisfied with the natural color found in keratin materials such as in human hair and have sought preparations capable of modifying their color. Numerous color modifying materials and many varied procedures for applying such materials have been developed in an attempt to formulate a coloring technique which will give good leveling properties and at the same time will have good uptake and fastness of the dye used.
Generally, there are three methods for dyeing fibers of all kinds. These being absorption, aggregation and bonding techniques. In the absorption method the dyestuff or the vehicle in which it is introduced has an electron affinity for the fiber being treated. This technique does not include true chemical bonding of dye to the fiber but relies primarily on the charge differential between the materials.
The aggregation method of dyeing involves a system of insolubility which is created inside the fiber to be dyed. The dyestuff is made fast by penetration into the fiber and the precipitation therein of the chromogenous agent. Textile type dyestuffs are normally applied by this method. In the bonding technique, a chemical bond between the dye material and the fiber is produced. Fiber reactive dyestuffs are applied by this method which makes use of a reaction between some constituent of the dye molecule and a reactive group of the fiber such as a thiol or amino group.
The use of the absorption or aggregation methods of dyeing keratin fibers has several disadvantages. One of the main disadvantages is the lack of fastness in these methods since there is no true bonding of the chromogenous agent to the fiber. It is to be observed that in a purely aggregative technique, for example, as where precipitation is employed, there cannot be bonding and the only reason the dye has any fastness at all is that some penetrates into and is left within keratin fiber which has been swollen during the treatment. Where absorption techniques are utilized with oxidative dyes, there is still no true bonding since the colored complex is not bound to the keratin fiber by true covalent bonds. The lack of permanency of the dyes used with this technique attests to its lack of fastness.
The bonding technique wherein reactive dyes are utilized has shown, in general, the properties of good uptake and fastness. The chemical bond which is formed between the dye material and the treated fiber forms a truly permanent dyed material so that some subsequent process must be performed to cause removal of the color. In the past the use of reactive dyes as a chromogenous material was limited to textile materials and the like due to the severe conditions which were needed to force the reaction to occur. More recently, reactive dyes of the class of halotriazinyl compounds have been found to be reactive with fibrous materials at less harsh conditions than were previously necessary. These dyes are, therefore, potentially useful in the cosmetic industry as colorants for human hair, but up to the present time the halotriazinyl dyes have not been used widely due to poor leveling properties. Samples made with these dyes have, in general, formed uneven coloring.